The invention relates to channel indicators for radio receivers of the type which receive a high-frequency signal which is mixed with an oscillator frequency generated in the radio receiver, the difference frequency between the high frequency and the oscillator frequency in the resultant frequency spectrum being processed further.
In general, radio receivers are provided with an indicator panel on which are indicated the names of a series of transmitting stations or their frequencies, usually referred to simply as "channels". Such an indicator panel is utilized to help tune in a desired channel or to indicate which channel has just been tuned in. Conventional indicator panels are comprised of a scale across which an indicator pointer can be moved by rotating a turning knob. The position which the pointer occupies at any given time indicates which channel has been selected. The movement of the indicator pointer is coordinated with the tuning mechanism of the receiver, so that the pointer moves as the frequency to which the receiver is tuned is being adjusted.
Besides this extremely well known, purely analog type of channel indicator, increasing use has lately been made of digital channel indicators. In these, the numerical value of the frequency of the selected channel is digitally displayed on an indicator. This permits the user to see with a glance the exact frequency of the selected station.
A disadvantage of this more recent, purely digital type of channel indicator is that it does not provide the user with an immediately and sufficiently clear indication of which channel has been selected, because the digitally displayed frequency value cannot be interpreted without knowing the correlation between the available stations (e.g., their call letters) and their exact frequencies. The average user of a radio receiver cannot be expected to remember the frequencies of all channels in his area, or even the frequencies of those stations to which he will most often tune in. In contrast, with the more usual analog-type station indicators, the user is able to determine what station has been selected merely from the spatial position of the indicator pointer.
The relationship between pointer position and channel is particularly simple in, for example, European ultrashortwave radio transmission, because station frequencies are uniformly spaced apart at intervals of 100 KHz.
The wavelength range of ultrashort radio waves extends from 1 to 10 meters, corresponding to a frequency range extending from 300 down to 30 MHz. This range was subdivided into bands allocated to different practical uses, by the 1947 Atlantic City World Communications Conference. Today, in the Federal Republic of Germany, for example, the 3-meter band (90-MHz band), extending from 3.0 meters to 3.43 meters and from 100 MHz to 87.7 MHz, is utilized for radio broadcast purposes. The frequency spacing between ultrashort-wave transmitters was at first set at 400 kHz, and after July 1, 1953 at 300 kHz. After Sept. 1, 1962, the frequency spacing was reduced to 100 kHz. This progressive decrease in inter-station frequency spacing was in part predicated upon the limited transmission range of ultrashort waves. The allocation of frequencies to broadcast transmitters takes into account their transmission ranges and their distances from one another. Broadcast transmitters not located far enough from one another to preclude interference are, as before, assigned frequencies spaced apart by frequency intervals of 300 kHz. The original denomination of transmitting stations by means of channel numbers has been retained; however, the additional stations newly introduced at frequencies 100 kHz below and 100 kHz above each original station are denominated by the same channel number as the original station, but with the addition of a "+" or a "-". Hereinafter, the channels spaced apart by frequency intervals of 300 kHz are referred to as "main channels", whereas the more recently introduced intermediate channel associated therewith are referred to as "left side channels" and "right side channels".
Thus, for some time now, the frequency spacing between adjoining stations in the ultrashort-wave range has been constant. In contrast, until quite recently, similarly regular relationships have not characterized the long-wave and medium-wave radio transmission ranges.
However, the new wavelength-distribution plan of Nov. 22, 1975, established for Europe, Africa, Asia and Oceania now provides for greater uniformity in the frequency spacing of broadcast transmitters. This new allocation system provides that the medium-wave range be subdivided into 120 channels having an interchannel spacing of 9 kHz, and that the long-wave range be divided into 15 channels likewise having an interchannel spacing of 9 kHz (cf. "Funkschau," 1974, vol. 17, p. 655). Thus, uniform interchannel spacings will characterize not only the ultrashort-wave range, but the medium-wave and long-wave ranges as well, the interchannel spacing for the medium- and long-wave ranges now being 9 kHz and for the ultrashort-wave range 100 kHz.
As a result of this new allocation, the use of analog channel indicators begins to offer considerable advantages relative to digital channel indicators, inasmuch as non-linearities in interchannel spacing have been eliminated.
In view of the above, it would appear to be particularly advantageous to provide a radio-receiver channel indicator having the characteristics of analog channel indicators, in order to provide a simple visual overview of channel selection, but also having the characteristics of digital channel indicators, for the sake of precision.
It has already been proposed ("Radio Mentor," vol. 10, 1971, p. 605) to provide a channel indicator comprised of a light-emitting-diode scale. That indicator included a row of 16 light-emitting diodes, of which only one is illuminated at any given time, the position of the illuminated diode indicating the frequency to which the receiver is tuned.
It has also been proposed ("Elektor," April 1973, p. 21) to provide another radio receiver with an electronic front panel, likewise comprised of light-emitting diodes which indicate the selected frequency. In this proposal, the individual light-emitting diodes are switched on and off in coordination with the application of tuning voltages to a varactor tuner.
It is also known (U.S. Pat. No. 3,898,642) to provide an analog indicator arrangement which is controlled by digital signals. This arrangement includes a plurality of light-emitting elements arranged in a row. However, this arrangement is intended primarily for the indication of electrical voltages, and the like, and cannot be readily incorporated into a radio receiver for channel indication purposes.
German Offenlegungsschrift DT-OS No. 2,435,088 discloses a frequency indicator for radio receivers of the type in which the tuned frequency is adjusted using voltage-dependent impedances. The frequency indicator comprises A.C.-operated and digitally controllable liquid-crystal elements which are arranged in the form of a scale.
The disadvantage of all these known pointwise channel indicators is that they are based upon circuit concepts which lead to a relatively high degree of inaccuracy or imprecision and are furthermore very susceptible to drift phenomena. Quarz-crystal precision and stability cannot be achieved using the circuit concepts of these known indicator arrangements.
It has also been proposed ("Radio-TV-Electronics," 1973, No. 8, "Revox A 700") to provide a digitally controlled channel-frequency indicator comprising a frequency synthesizer. With this proposed system, the control of the "program counter" of the system is effected using diode matrices for the MHz and kHz frequency settings. These matrices simultaneously serve to control the operation of a 5-place nixie-tube channel-frequency indicator. When this indicator is used to provide a numerical channel-frequency indication, the advantages inherent in analog indicators and described above are lost. Additionally, the circuit concepts underlying this proposed system involve considerable additional expense for code converters and for the means for establishing proper resolution for the channel-frequency settings (medium-wave and long-wave).
Frequency synthesizers are in themselves well known (see for example "Funkschau," 1973, vol. 3, pp. 85-87); and German Offenlegungsschriften Nos. 2,328,011; 2,142,779; 2,164,175; 2,123,736; 2,237,824; 2,217,002; 2,441,809 and 2,122,658). In the simplest case, they comprise an oscillator tunable by means of a D.C. voltage. The oscillator furnishes an A.C. voltage in the range of the desired output frequencies. A frequency divider frequency-divides the oscillator voltage and then applies it to a phase-comparison circuit and also to a low-pass filter, through the intermediary of which the output voltage of the phase-comparison circuit is fed back to the oscillator. Accordingly, a frequency synthesizer operates as a phase-coupled device for the various frequencies (phased-locked loop), and makes it possible to derive a large number of tuning frequencies from only a single quarz-crystal-stabilized frequency (cf. "Funkschau," 1973, vol. 9, pp. 325-327; vol. 10, pp. 361-364; "Funkschau," 1974, vol. 2, pp. 62-63; vol. 3, pp. 93-95; "Funkschau," 1974, vol. 6, pp. 177-180).